1. Field of the Invention
This invention relates to charge transfer devices, and in particular to transversal filters manufactured from charge coupled devices in which the tap weights are achieved with a network of mask programmable capacitors.
2. Description of the Prior Art
The use of charge transfer devices in transversal filters is known. See, e.g. "Transversal Filtering Using Charge-Transfer Devices, by Buss et al., IEEE Journal of Solid-State Circuits, Vol. SC-8, No. 2, April 1973, pgs. 138-146. As described by that article, a transversal filter usually consists of a sampling stage followed by a series of delay stages, each of which delays the signal by a number of clock periods. As the signal passes each delay stage it is non-destructively sampled and an appropriate weighting coefficient is applied. The weighted signals from the delaying stages are then summed together to give the filter output. The theory and operation of such devices is further discussed in that article.
In a number of laboratories throughout the world efforts have been made to achieve practical charge coupled device (CCD) transversal filters for a variety of signal processing applications. Very high performance fixed-tap-weight filters have been developed which illustrate the utility of such filters. For example, a monolithic D3 filter for a telephone CODEC was reported in "A PCM Voice CODEC with On-Chip Filters," by J. Terry et al., Digest of the 1978 International Solid-State Circuits Conference, pg. 182.
A number of programmable analog tap-weight filters have also been developed. Unfortunately these have all required a large number of near real-time inputs which necessitate many connections to the integrated circuit, a large memory, or both. Although these complex systems may be practical for certain multifunction systems where the charge coupled device may be re-programmed from one filter function to another, they are rarely practical where a fixed, single-function is required.
Many system designers have sought small integrated circuit filters to provide specific spectral characteristics, yet need them only in moderate quantities on the order of less than a thousand pieces, at prices of less than one hundred dollars each. Such potential users can rarely justify the cost or development time of a custom integrated circuit chip program.
One prior art attempt to meet this demand is described in "An Electrically Programmable CCD Transversal Filter with Variable Capacitance Weight Factors," by H. Wallinga and I. Hylkema, IEEE Journal of Solid-State Circuits, Vol. SC-14, No. 3, June 1979, at pgs. 538-542. The approach described in that article involves electrically programming the tap-weights through voltage-dependent capacitors. To bias these voltage-dependent capacitors to a desired value, that technique relies upon an additional capacitor to de-couple the reset bias of the floating gate sense amplifier and the tap-weight reference voltage.